The field of automotive emission controls represents an area which is receiving a great deal of attention, particularly as government regulations concerning automotive pollution become more and more strict. It is now accepted that some sort of catalytic control system will be necessary in order that automobiles may meet future regulations, particularly with regard to emissions of oxides of nitrogen.
Past research has demonstrated that noble metals such as platinum and palladium are among the most effective known catalysts for the diminution of automobile exhaust gases. Unfortunately, these metals are extremely expensive and it is necessary that some means be derived to minimize the amount of noble metal used while maximizing the surface area. The obvious solution is to apply a very thin layer of noble metal to a substrate. Substrates which have been experimentally used include ceramics such as ceramic honeycombs and metal alloys. The ceramic substrates suffer from the defect of being excessively brittle and are susceptible to both thermal and mechanical shock. Attempts to apply a thin noble metal layer to a metallic substrate have generally been unsuccessful because at the high temperature encountered in operation, the noble metal soon diffuses into the substrate losing its effectiveness.